Have you ever been in a situation where you had to enter an email service’s Simple Mail Transfer Protocol (SMTP) port number? What number did you specify? 25? 587? 465? All three TCP port numbers are associated with SMTP. SMTP differs from other network protocols in a way that it’s associated with multiple port numbers. Most network protocols are only associated with one. For example, FTP = 21, SFTP = 22, HTTP = 80 and so on.